Information regarding location of a source such as for surveillance or combat search and rescue can be degraded in value if detected by unfriendly entities, such as enemy forces in the case of a downed pilot or a marked terrorist under surveillance.
Intentional detection of the signal or message can be accomplished in military systems that use specially designed electronic support measures (ESM) receivers. These ESM receivers are often found in signal intelligence (SIGINT) applications. In commercial applications, devices employed by service providers (i.e. spectral monitors, error rate testers) can be used to detect intrusion on their spectral allocation. Inadvertent detection can also occur, such as when a user or service provider notices degradation in link performance (e.g., video quality, audio quality, or increased bit error rate).
The term covert also implies the additional goals of evading interception and exploitation by unintended receivers. Interception is the measurement of waveform features or parameters useful for classifying/identifying a transmitter and/or the waveform type and/or deriving information useful for denying (i.e. jamming) the communication. Exploitation is processing a signal by an unintended receiver in the attempt to locate the transmitter and/or recover the message content. In the broad literature on covert communications these characteristics as applied to transmitted information signals are referred to as low probability of detection (LPD), low probability of intercept (LPI), and/or low probability of exploitation (LPE) by an unintended receiver.
Given the desirability to transmit messages covertly, it is helpful to understand considerations that enhance or degrade LPD, LPI and LPE. An unintended receiver such as the receiver 103 in FIG. 1 with the goal of detecting a covert communication must reliably differentiate between the binary noise-only and signal-plus-noise hypothesis. As is known to those of skill in the art, for an unintended receiver the signal detection process is typically based on an energy threshold. The energy the receiver measures is given by Etot=PavgTxmit. Where under general conditions the power Pavg is the received covert signal power S plus internal receiver noise power N. Hence, Etot=(S+N)Txmit. If the signal power used to communicate is only a small fraction of the receiver noise, S<<N, it is extremely difficult for the unintended receiver to reliably detect the presence of the covert signal because the total energy detected will only be marginally greater than the noise-only (S=0) case.
Minimizing transmit power has two direct system benefits. First, the total signal power used will be a small fraction of the total noise power present in the same band. Thus, if the message is limited in time duration, the total energy measured by an unintended receiver 106, which may be an ESM receiver, is indistinguishable from a noise-only environment. Since ESM receivers are often of energy threshold type, there is an obvious trade-off of average power for time duration in order for a signal to remain undetectable. Second, the low transmit power scenario enables usage by transmitters with very limited power supplies (i.e. batteries).
Therefore, as naturally arise in military environments such as depicted in FIG. 2, there is a need for a low power message system and method, covert or otherwise, such as covert communications for Intel or Special Forces, “stealth” IFF for low observable ground vehicles, and combat search and rescue (CSAR). There is also such a need in a number of civilian or public safety applications as well, such as asset tracking/location or “lost child” detection/location and surveillance. In particular in these latter-described applications it may be particularly desirable to receive both a message and location the source of the message.
As mentioned above it is often of interest to geolocate signals, particularly those that may de designed for LPI/LPS. These included spread spectrum signals, spread spectrum a signals are intentionally low power as previously discussed, and these signal can also be co-channel with many other signals of similar type, which makes geolocation by prior art methods and systems ineffective.
Embodiments of the present inventive system and method address the above needs while requiring only an extremely low power signal. The geolocation needs are specifically addressed by estimating two cumulant matrices, and performing generalized eigenvalue decomposition (GEVD) of the resulting matrix pencil. The GEVD provides eigenvectors orthogonal to the incoming steering vectors, save one. Exploiting this property allows estimating of steering vectors for each incoming signal. From the steering vectors it is easy to arrive at AOA or geolocation. The embodiments enable geolocation signals that are below thermal noise and in co-channel environments.
These and other advantages of the disclosed subject matter will be readily apparent to one skilled in the art to which the disclosure pertains from a perusal or the claims, the appended drawings, and the following detailed description of the preferred embodiments.